The invention pertains to the general field of printing and in particular to inkjet printing.
Ink jet technology may be used to deposit fluid materials on substrates. Ink jet technology has numerous applications, mainly in printing. Ink jet printers function by depositing small droplets of fluid at desired positions on a substrate. There are various ink jet printing technologies. Many of these technologies can be classified in two general categories. Continuous ink jet printing involves electrically charging a stream of droplets and then deflecting the stream directly or indirectly onto a substrate. xe2x80x9cDrop on demandxe2x80x9d (DOD) inkjet printing has an actuator connected to an ink supply. The actuator creates ink droplets on demand. The actuator may comprise, for example, a piezoelectric actuator.
Ink jet printing suffers form a number of drawbacks. Ink jet printing is typically slower than traditional offset printing. This is especially true for process color printing. For example, the linear printing speed of inkjet printing is typically of the order of 10 times slower than can be achieved in offset printing. This represents a major issue limiting the implementation of inkjet technology in industrial printing systems. The inkjet printing speed limit is dictated by the rate at which inkjet nozzles can eject ink in discrete controllable amounts. This rate is at present on the order of 20,000 pulses per second for DOD inkjet printers. This limits state of the art DOD inkjet printers to print rates on the order of 2 pages per second. Continuous ink jet printing can be performed more quickly. However, at high speeds, the results tend to be poor. Quality may be improved by printing at slower speeds.
Inkjet printing typically cannot achieve printing quality as high as can be achieved using offset printing techniques. Inkjet printing is often characterized by a distinctive banding pattern that is repeated over the printed image. This may be traced to the arrangement of the inkjet nozzles in the printing head. Relatively small nozzle misalignments or off-center emission of droplets can cause banding. As the printing head is translated laterally across the width of the printing surface, the visual imperfections are periodically repeated. This produces banding or striping which is characteristic of inkjet printers. A number of approaches exist to control banding. These approaches reduce throughput of the printer.
Print quality of inkjet printers is also reduced by xe2x80x9cwickingxe2x80x9d or xe2x80x9crunningxe2x80x9d. The low-viscosity water-based inks typically employed in ink-jet printers tends to xe2x80x9crunxe2x80x9d along the fibers of certain grades of paper. This phenomenon is also referred to as xe2x80x9cwickingxe2x80x9d and leads to reduced quality printing, particularly on the grades of paper employed in volume printing. Wicking can cause printed dots to become much larger than the droplet of ink emerging from the inkjet nozzle.
It is possible to reduce wicking by printing on specially treated paper. However, such paper tends to be undesirably expensive.
The matter of failure in inkjet nozzles is also deserving of attention. Various approaches exist for detecting faulty inkjet nozzles and for re-addressing the inkjet printing head to permit other nozzles to perform the tasks of faulty nozzles. This includes various redundancy schemes. Again, these usually have the effect of slowing down the net printing process speed. In many cases the redundancy is managed at printing head level, requiring backups for entire printing heads. This adds to the cost of the technology per printed page and again limits the industrial implementation of the technology.
The prior art describes various array inkjet print head designs aimed at reducing inkjet-printing artifacts such as banding. Examples are Furukawa in U.S. Pat. No. 4,272,771, Tsao in U.S. Pat. No. 4,232,771, Padalino in U.S. Pat. No. 4,809,016 and Lahut in U.S. Pat. No. 5,070,345. Considerable work has also been done in addressing reliability by providing inkjet nozzle redundancy. Examples are Schantz in U.S. Pat. No. 5,124,720, Hirosawa in U.S. Pat. No. 5,398,053 and Silverbrook in U.S. Pat. No. 5,796,418. Transfer rollers have also been described, both with and without the droplets deposited on them being processed in some way before final printing in order to reduce wicking. See for example Takita in U.S. Pat. No. 4,293,866, Durkee in U.S. Pat. No. 4,538,156, Anderson in U.S. Pat. No. 5,099,256, Sansone in U.S. Pat. No. 4,673,303 and Salomon in U.S. Pat. No. 5,953,034.
There is a need for inkjet printing methods which provide combinations of print quality, speed and cost which improve on the prior art.
This invention, provides an inkjet printing method in which inkjet droplets are deposited onto an intermediate transfer surface. On the transfer surface the droplets are treated to decrease their sizes and to increase their viscosities. The treated droplets are then transferred to a printing surface. Dots immediately adjacent to one another in the pattern may be printed in separate passes to retain dot integrity. The droplets may comprise droplets of a UV-curable material and the treatment may comprise exposing the droplets to ultraviolet light while on the transfer surface. The transfer surface may optionally be patterned.
Further aspects of the invention and features of specific embodiments of the invention are described below.